Utils: Fix memory leak in JsonMemoryPool

objects are freed, but their dtors are not called...

Detected by valgrind

Change-Id: Iec8d78c205e9b2722ee5a89941df96c3383b385b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>

src/libs/utils/json.cpp

@@ -42,8 +42,10 @@ using namespace Utils;
 
 JsonMemoryPool::~JsonMemoryPool()
 {
-    foreach (char *obj, _objs)
+    foreach (char *obj, _objs) {
+        reinterpret_cast<JsonValue *>(obj)->~JsonValue();
         delete[] obj;
+    }
 }
 
 JsonValue::JsonValue(Kind kind)